Some of the most commonly used curing systems for curable polymers (including elastomers) are commonly classified as either sulfur curing systems or peroxide curing systems. Peroxide curing systems initiate crosslinking via free radical mechanisms, using as the curing agent a compound which is frequently described as an organic peroxide. Advantages of peroxide curing systems include: ability to cause crosslinking in lower compression set, better chemical and oil resistance, less discoloration, and better color stability with less staining. Problems associated with peroxide curing systems include scorch (premature crosslinking) and poor aging performance.
Coagents have been used in peroxide curing systems in order to ameliorate the disadvantages while capitalizing on the desirable characteristics of peroxide curing systems. By and large, these coagents are low molecular weight monomers and are usually either methacrylates, acrylates, cyanurates or isocyanurates.
Coagents usually increase crosslink density, which results in an increase in the cure state of a peroxide cured elastomer. It is therefore often possible to lower the amount of peroxide required to achieve a given state of cure. A coagent typically does not affect the rate of cure. On the negative side, a coagent may reduce scorch time.
Rubber & Plastics News. Feb. 20, 1989, pages 40-43, describes the use of organic peroxides for crosslinking elastomers. Various organic peroxides and peroxyketals are listed and discussed, as are representative polymers which can be crosslinked with the aid of an organic peroxide. This article also briefly discusses the effect of a coagent; among the coagents (or co-curing agents) mentioned is trimethylol propane trimethacrylate (TMPTMA).
Rubber & Plastic News, Sept. 2, 1991, pages 37-40, is directed primarily to scorch safety in coagent cured elastomers. As stated therein, coagents are used to improve processability and to enhance the physical properties of peroxide-cured elastomers. Coagents are described as low molecular weight monomers which behave as reactive plasticizers and lower the viscosity of the compound mixture. This article notes that they increase crosslink density, thereby often lowering the amount of peroxide required. On the other hand, they do result in shorter scorch time as the article notes. Representative coagents mentioned are 1,3-butylene glycol dimethacrylate (BGDMA), trimethylolpropane trimethacrylate (TMPTMA), zinc diacrylate (ZDA), and zinc dimethacrylate (ZDMA).
U.S. Pat. No. 3,751,378 to Cowperthwalte, et al, describes peroxide curing of elastomers in which a polyfunctional liquid aliphatic acrylate or methacrylate is employed as a coagent. One of the coagents is TMPTMA (SR.350). Mooney scorch time is improved by including in the curing recipe about 0.3-3 parts of either an N-nitroso diarylamine (N-nitroso diphenylamine is specifically illustrated) or N,N'-dinitroso diphenyl-p-phenylene diamine.